Chemistry – A European Journal, 14 March 2016, Vol.22(12), pp.4087-4099
The intrinsic bond strength of C in its Σ ground state is determined from its stretching force constant utilizing MR‐CISD+Q(8,8), MR‐AQCC(8,8), and single‐determinant coupled cluster calculations with triple and quadruple excitations. By referencing the CC stretching force constant to its local counterparts of ethane, ethylene, and acetylene, an intrinsic bond strength half way between that of a double bond and a triple bond is obtained. Diabatic MR‐CISD+Q results do not change this. Confinement of C and suitable reference molecules in a noble gas cage leads to compression, polarization, and charge transfer effects, which are quantified by the local CC stretching force constants and differences of correlated electron densities. These results are in line with two π bonds and a partial σ bond. Bond orders and bond dissociation energies of small hydrocarbons do not support quadruple bonding in C. : Multireference‐coupled cluster calculations of free and confined dicarbon, C, lead to a bond strength order of 2.5 relative to that of ethane according to adiabatic/diabatic calculations and local stretching force constants. Electron density and energy density together with Wiberg and Mayer bond indices, all determined at the multireference level, suggest a bond multiplicity lower than 3 in line with the bond strength order.
Ab Initio Calculations ; Bond Strength ; Carbon ; Force Constants ; Quadruple Bonds ; Space Confinement Analysis